Wild chimpanzee behavior suggests that a savanna-mosaic habitat did not support the emergence of hominin terrestrial bipedalism

Bipedalism, a defining feature of the human lineage, is thought to have evolved as forests retreated in the late Miocene-Pliocene. Chimpanzees living in analogous habitats to early hominins offer a unique opportunity to investigate the ecological drivers of bipedalism that cannot be addressed via the fossil record alone. We investigated positional behavior and terrestriality in a savanna-mosaic community of chimpanzees (Pan troglodytes schweinfurthii) in the Issa Valley, Tanzania as the first test in a living ape of the hypothesis that wooded, savanna habitats were a catalyst for terrestrial bipedalism. Contrary to widely accepted hypotheses of increased terrestriality selecting for habitual bipedalism, results indicate that trees remained an essential component of the hominin adaptive niche, with bipedalism evolving in an arboreal context, likely driven by foraging strategy.


Fig. S1.
Inter-site differences in principle locomotor category frequencies. Bars represent frequency of quadrupedal walking (green), climbing (blue), and suspension (yellow) (see Table S1 for category definitions). The amount of time spent walking quadrupedally correlates negatively with time spent in the trees. Note that Issa chimpanzees spend less time in quadrupedal walking, and more time engaging in solely-or primarily-arboreal behaviors (i.e. suspension and climbing) than all sites except Kibale. Additionally, Issa chimpanzees spend a surprisingly high proportion of locomotion in suspensory behaviors, a terminal branch behavior that is associated with bipedalism as a safe way to move in thin branches (41,69) (see also Fig. S2). Locomotor categories 'Bipedal' and 'Other' are here not included due to being very rare behaviors (<2% across sites). Mahale, Gombe, and Taï data from (36), Kibale data from (37). Schematics sourced from (73). Inter-site differences in arboreal locomotor category frequencies. Bars represent frequency of quadrupedal walking (green), climbing (light blue), suspension (yellow), and bipedalism (dark blue) (see Table S1 for category definitions). When in the trees, Issa chimpanzees engaged in more suspensory locomotion and climbed less, but used a similar amount of bipedalism compared to other chimpanzee communities with available arboreal locomotor data (sourced from (36), Taï values estimated from male and female averages). Arboreal locomotor frequencies are not available from Kibale. Once in the trees, there was no significant difference in locomotor behavior between the forest and woodland vegetation types or sexes at Issa, considering main locomotor modes. 'Other' category was not included here due to being a rare behavior (<2% across sites). Schematics sourced from (73).  Table S1. Classification of locomotor and postural behavior following refs. (37,68,69). Categories are used as broader groupings to allow meaningful comparison to past studies (36). Majority of weight is borne by one or two forelimbs, grasping an above support. Elbows and shoulders fully extended, torso is orthograde. Pronograde suspend Majority of weight is borne by both forelimbs and hindlimbs grasping a support above the animal. Torso is horizontal.

Hindlimb suspend
Hanging upside down by the feet/foot, may swing.

Lie
Majority of weight is borne by the torso, in a horizontal position on a roughly horizontal support. Cling All four or three limbs are flexed and hands and feet grasp to (sub-) vertical support(s). No weight is borne on the ischia. Torso is near orthograde, exception in infants.
*In analyses, bipedal pools postural and locomotor observations, unless indicated otherwise.
ᵻ In more recent categorization schemes these modes would be grouped in the 'Other' locomotor category (e.g., (37,69)), but are here grouped in categories following (36) to permit intersite comparison of general locomotor behavioral profiles.  Table S3.
Definitions used for other variables collected as part of this study (activity and substrate type). All activity states not in bold font were grouped together as "other" for this study, as they were rare or not involved in locomotor behaviors.

ACTIVITY STATE Definition feeding
Involves foraging and food-processing with or without using a tool traveling Movement not associated with a specific activity such as foraging, feeding etc. grooming Involves allo-grooming and self-grooming with or without using a tool.    * Only locomotor bipedalism was recorded in this study, with bipedal stand subsumed with quadrupedal stand in category "stand", so bipedal stand is not included in the number of bipedal observations and frequency is likely largely underestimated. **likely biased in arboreal observations due to low habituation and ground visibility.